The Emergence of a vv + MDV Can Break through the Protections Provided by the Current Vaccines.

Yan-Li Gao,Qiao-Mu Deng,Teng Huang,Min Li,Pei-Kun Wang,Ping Wei,Qiu-Hong Li,Wei-Wei Wang,Meng-Ya Shi

doi:10.3390/v12091048

Abstract

Marek’s disease (MD) is an infectious malignant T-cell lymphoma proliferative disease caused by Marek’s disease virus (MDV). In recent years, the emergence of very virulent (vv) and/or very virulent plus (vv +) strains of MDV in the field has been suggested as one of the causes of vaccination failure. The pathogenicity of the MDV strain GX18NNM4, isolated from a clinical outbreak in a broiler breeder flock that was vaccinated with CVI988/Rispens, was investigated. In the vaccination-challenge test, GX18NNM4 was able to break through the protections provided by the vaccines CVI988 and 814. It also significantly reduced body weight gain and caused marked gross lesions and a large area of infiltration of neoplastic lymphocyte cells in the heart, liver, pancreas, etc. of the infected birds. In addition, the expressions of programmed death 1 (PD-1) and its ligand, programmed death ligand 1 (PD-L1), in the spleens and cecal tonsils (CTs) of the unvaccinated challenged birds were significantly increased compared to those in the vaccinated challenged birds, indicating that the PD-1/PD-L1 pathway is related to immune evasion mechanisms. The results showed that the GX18NNM4 strain could cause severe immunosuppression and significantly decrease the protections provided by the current commercial vaccines, thus showing GX18NNM4 to be a vv + MDV strain.

Highlights

Marek’s disease (MD) is a lymphoproliferative disease of chickens caused by the highly infectious cell-associated alpha-herpesvirus, classified as Marek’s disease virus (MDV) [1]
We investigated the pathogenicity of MDV strain GX18NNM4, isolated from a breeder flock of Yellow chickens in southern China that had been vaccinated with CVI988/Rispens at hatching and had experienced depression, weakness, reduction in weight gain and an increased death rate after 120 days of age
It has been reported that there are many clinical tumor cases caused by highly virulent MDV strains in MD-vaccinated chicken flocks [21,27,33,34,35]

Summary

Introduction

Marek’s disease (MD) is a lymphoproliferative disease of chickens caused by the highly infectious cell-associated alpha-herpesvirus, classified as Marek’s disease virus (MDV) [1]. Only MDV-1 is pathogenic and causes tumors in susceptible chickens. Since MD was first reported in 1907 [2], MDV has spread rapidly around the world, causing clinical disease, increased mortality and reduced growth, as well as sub-clinical immunosuppression, decreasing antibody levels of vaccinations [3,4]. MD has been well controlled by the use of MDV attenuated vaccines. These include the antigenically related herpesvirus of turkeys (HVT), MDV-2 strain SB-1 as well as the widely used MDV-1 strains CVI988/Rispens and 814 [6,7]. CVI988 and 814 are Viruses 2020, 12, 1048; doi:10.3390/v12091048 www.mdpi.com/journal/viruses

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